1. Field of the invention
The present invention relates, in general, to a process for preparing water-in-oil type emulsion cosmetic compositions containing L-ascorbic acid and, more particularly, to an improvement in the stability in cosmetic compositions and the whitening activity of L-ascorbic acid, along with the process.
2. Description of the Prior Art
In recent years, active research has been directed to the development of cosmetic whitening agents and whitening cosmetics. Particularly, the whitening cosmetic market of Asia has yearly increasingly explosive growth.
The mechanism of whitening agents is largely believed to follow one of the following three courses:
(1) melanin reduction and depigmentation;
(2) inhibition against tyrosinase activity; and
(3) control of cytokine network.
Oxidized melanin shows dark colors. When being reduced, melanin is decolorized to pale yellow. Well known as melanin reducing agents are vitamins, including tocopherol, L-ascorbic acid, and derivatives thereof. Of them, L-ascorbic acid draws great attention because it is found to have an inhibitory activity against tyrosinase in addition to showing depigmentation. Further, with a high degree of safety to the human body, L-ascorbic acid is used as a thermally useful material in a broad spectrum of cosmetics.
L-ascorbic acid, however, suffers from a disadvantage of being easily oxidized and degraded by moisture or acid. That is, when brought into contact with moisture or acid, L-ascorbic acid loses its effective activity as well as causes yellowing and gives out an offensive odor.
Accordingly, stabilization of L-ascorbic acid has been a hot issue in the art. As a consequence of continual research, various L-ascorbic acid derivatives, which are stable to acid and moisture, and protective systems such as liposomes are developed. In detail, Japanese Pat. Laid-Open Publication No. Heisei 2-83309 discloses a water-in-oil type emulsion which contains water-soluble L-ascorbic acid derivatives in its inner phase so as to protect them from oxidation and degradation. Japanese Pat. Laid-Open Publication No. Showa 55-64511 suggests an inclusion of higher fatty acid esters of L-ascorbic acid in cyclodextrin. Japanese Pat. Laid-Open Publication No. Heisei 3-5426 introduces a liposome concept in improving the preservatory stability of L-ascorbic acid, teaching that L-ascorbic acid phosphate or sulfate is added in a liposome. In Japanese Pat. Laid-Open Publication No. Heisei 5-345714, there are disclosed certain water-soluble polymers which can be formulated with inorganic acid ester of L-ascorbic acid.
All of the above-cited references utilize ester derivatives of L-ascorbic acid. Improved as they are in stability compared with L-ascorbic acid itself, the L-ascorbic acid derivatives are found to show poor melanin reduction or minute inhibitory activity against tyrosinase.
The intensive and thorough research on the use of L-ascorbic acid in cosmetics, repeated by the present inventors aiming to improve the stability as well as the functionality of L-ascorbic acid, resulted in the finding that the use of 2-hydroxypropyl cyclodextrin and a water-in-oil type emulsion allow L-ascorbic acid itself to be preserved for a lengthy period of time with high functional maintenance.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a water-in-oil type emulsion cosmetic composition containing L-ascorbic acid, which can preserve L-ascorbic acid itself for a lengthy period of time as well as keep the function of L-ascorbic acid high.
It is another object of the present invention to provide a method for preparing such an L-ascorbic acid-containing, water-in-oil type emulsion cosmetic composition.
In an embodiment of the present invention, a water-in-oil type emulsion cosmetic composition containing L-ascorbic acid with improved stability is prepared according to a method comprising the steps of: (1) formulating L-ascorbic acid at an amount of 1-8 weight % with a 1:1 mix of 2-hydroxypropyl cyclodextrin and pure water while stirring at room temperature for 12 hours or longer to give an L-ascorbic acid base composition; (2) admixing the L-ascorbic acid base composition with a water-soluble component comprising 10-60 weight % of polyol to give an aqueous phase system; (3) mixing 2-4 weight % of a block-copolymeric non-ionic surfactant, 1-2 weight % of an ordinary non-ionic surfactant and 1-5 weight % of polyoxyethylene methylpolysiloxane at 65-75xc2x0 C. to produce an emulsion composition; (4) admixing 10-20 weight % of hydrocarbon oil with the emulsion composition to give an oil-phase system; and (5) mixing the aqueous phase system with the oil-phase system with stirring. In an aspect of the present invention, the 2-hydroxypropyl cyclodextrin is used at an amount of 5-25 weight % based on the weight of the water-in-oil type emulsion cosmetic composition.
The present invention is characterized in that an L-ascorbic acid base formulation comprising 1-8 weight % of L-ascorbic acid and a mixture of 1:1 2-hydroxypropyl cyclodextrin and pure water is admixed with an aqueous phase component composition and then with an oil-phase component composition.
The aqueous phase component comprises 10-60 weight % of polyol based on the weight of the water-in-oil type emulsion cosmetic composition.
In the water-in-oil type emulsion cosmetic composition, 2-hydroxypropyl cyclodextrin amounts to 5-25 weight % and pure water is added to a final volume of 100 weight %.
As for the oil-phase component composition, it comprises 2-4 weight % of a block-copolymeric non-ionic surfactant, 1-2 weight % of an ordinary non-ionic surfactant, 1-5 weight % of a polyoxyethylene methylpolysiloxane copolymer, and 10-20 weight % of a hydrocarbon oil, based on the weight of the water-in-oil type emulsion cosmetic composition. To prepare the oil-phase component composition, a block-copolymeric non-ionic surfactant, an ordinary non-ionic surfactant, and polyoxyethyelene methyl polysiloxane are first mixed together at predetermined amounts at 65-75xc2x0 C. and then, with a hydrocarbon oil.
The block co-polymeric non-ionic surfactant has both a hydrophilic group and a hydrophobic group in its molecular structure, exemplified by polyethylene glycol (PEG)-30 dipolyhydroxy stearate. Useful as the ordinary non-ionic surfactant is polyglyceryl-2-diisostearate. With regard to the hydrocarbon oil, it may be selected from those that are usually used in water-in-oil type emulsion cosmetic compositions. Examples of useful hydrocarbon oil include squalene and cetyl octanoate.
In accordance with the present invention, L-ascorbic acid is formulated with a mix of 1:1 2-hydroxypropyl cyclodextrin and pure water and the resulting formulation is stirred at room temperature for 12 hours. This separate L-ascorbic acid base formulation assures the L-ascorbic acid of stability in the water-in-oil type emulsion cosmetic composition.
Even after being stored for a significant period of time, the water-in-oil type emulsion cosmetic composition of the present invention does not exhibit discoloration nor gives out offensive odor nor shows the separation of cosmetic ingredients. In addition, the cosmetic composition of the present invention keeps L-ascorbic acid high in the effective whitening activity, such as melanin reduction and inhibitory activity against tyrosinase.